I'd say it's a decent-quality WTF, in the sense that anybody unfamiliar with Groovy (me, for instance) is going to go "WTF?"

The fact that Remy is concerned with something as piffling as a variable name, when the very same line seems to need toString() to "convert" something that has apparently been extracted from a collection of String into, er, a String .... well, either this example is even more borked than it looks, or else three familiar letters spring to mind.

The first line could actually make sense, if the original coder wanted to append or remove elements from the list later on.

split() returns an array, so to modify its length, you need to convert it into a list.
That's why Arrays.asList() is there. However, you still can't modify the resulting ArrayList, because the instance is of a specific subclass that's unmodifiable.
If you want a "real" ArrayList, you've got to feed the result of Arrays.asList() to "new ArrayList<>()".

The real WTFs I see in the first line of this code are 1. that split() returns an array and 2. ArrayList doesn't have a varargs constructor, but you'd have to blame that on Java, not Groovy itself.

Another WTF is that this is meant to be an example for Groovy while it's not even using Groovy syntax.
Converted to Groovy, this code would look like this:

def items = string.split(",") as ArrayList
def itemOne = item[2]

Addendum 2017-07-31 07:28:
Edit: The last line is missing a line break after ArrayList.

Without knowing what else is being done with "items", it's hard to call this a big WTF... this is also almost 100% Java code (which is also perfectly valid groovy). The final toString() is unnecessary, but certainly not really deserving of a whole article.

Also itemOne might be perfectly reasonable, as the first useful item in a list of things, the first couple of entries could be unneeded identifiers and this field is actual what they are interested in. I think the WTF is that the WTF is not a WTF.

I have to write groovy occasionally for Jenkins integration. It is by far the most interesting programming language to try to google. My favorite is the time I was trying to connect to an SMB share from a Jenkins job, and it was a Linux host, so naturally I googled "groovy samba."